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May 24, 2010

Megafauna Demise Could Have Cooled The Climate

by Sam Savage

The extinction of mammoths and other megafauna that came after humans spread out across the New World may be one explanation of a sharp decline in global temperatures more than 12,750 years ago, researchers reported on Sunday.

Roughly a hundred species of grass-eating giants that once flourished on the North American landscape released massive quantities of methane during their lifetime. As a heat-trapping greenhouse gas, methane is 30 times more potent than carbon dioxide.

It may not have been enough to trigger large-scale global warming, but when all that gaseous output went extinct with the beasts, it contributed to a prolonged freeze known as the Younger Dryas cold event, the researchers argue.

If correctly theorized, the "Anthropocene epoch" -- the era of major human impact on Earth's climate system -- began not with the Industrial Revolution in the 1800s, but when the massive scourge of human predators filled the Americas more than 13,000 years earlier.

A trio of researchers, led by Felisa Smith of the University of New Mexico, made calculations that show how this theory might be fact.

Analyzing data on cows and other modern-day ruminants, the scientists estimated the total methane output of pre-historic megafauna at nearly 10 trillion grams (more than 11 million tons) per year.

Ice-core samples also reveal that an abrupt drop in atmospheric methane levels of 180 parts per billion by volume (ppbv) coincides with the extinction of these gas-releasing giants and the cold spell that followed.

Greenland ice cores studied from other periods show that a reduction in methane levels of 20 ppbv equals a reduction in temperature by about 1.8 degrees Fahrenheit.

"We find that the loss of megafauna could explain 12.5 to 100 percent of the atmospheric decrease in methane observed," the researchers said.

The study concluded that the extinction of the megafauna in North America "is the earliest catastrophic event attributed to human activity."

"We thus propose that the onset of the 'Anthropocene' [epoch] should be recalibrated to 13,400 years before present, coincident with the first large-scale migrations of humans into the Americas," the researchers said.